Integrated circuits (ICs) are fabricated on wafers. Commonly, these wafers are semiconductor materials, for example, silicon. Through efforts of research and development, the size of the transistors making up the ICs has decreased, and as a result voltage supplied to the transistors decreases.
An IC is commonly coupled to a voltage regulator that is part of a power delivery network for the IC. The voltage regulator converts power supply voltages to the lower voltages used by the IC. The voltage regulator ensures a predictable power supply is provided to the IC.
As transistors of the IC turn on and off, however, the load on the power supply changes rapidly, placing additional demand on the voltage regulator. The distance between the voltage regulator and the IC creates a long response time, preventing the voltage regulator from increasing power to the IC instantaneously, especially when the transistors switch on and off millions or billions of times each second. Decoupling capacitors provide additional stability to the power supplied to the ICs.
Decoupling capacitors attached in close proximity to the IC provide instantaneous current to the IC. As demand on the power supply changes rapidly, the capacitor provides additional power and can refill at a later time when the power demand decreases. The decoupling capacitor allows ICs to operate at the high frequencies and computational speeds desired by consumers. However, as the transistor sizes have decreased and transistor densities increased, finding area on the IC for decoupling capacitors has become difficult.
One configuration for decoupling the IC locates decoupling capacitors directly on the IC die. However, locating decoupling capacitors directly on the IC die occupies die area that could otherwise be used for active circuitry. Additionally, fabricating decoupling capacitors on the die involves additional manufacturing time that increases the cost of manufacturing.
As one example, a conventional decoupling capacitor used in ICs is a thin film capacitor. Thin film capacitors may be fabricated on the wafer at an additional cost during manufacturing. These capacitors are typically alternating layers of a dielectric material followed by a conductor. Although the thin film capacitor is a simple structure, the capacitance is determined largely by the number of series capacitances in parallel. As more capacitance is added, however, the thin film capacitor structure increases in height.
Metal-insulator-metal (MIM) capacitors may be manufactured to fit in smaller height constraints than thin film capacitors. When packaging the capacitors, height may be an important consideration. Furthermore, MIM capacitors offer additional flexibility over thin film capacitors in designing the equivalent series inductance (ESL) and equivalent series resistance (ESR) in the power delivery network.
As packages shrink in size to fit the smaller form factors present in mobile devices, for example, the space that is available on the package decreases. Additionally, as the circuits operate at higher frequencies, higher capacitances are required to ensure proper operation of the circuitry and transistors.
For example, as ICs operate at higher frequencies they are affected by the total characteristic impedance of the power delivery network. The total characteristic impedance is affected by the inductance of the traces in the decoupling capacitor (i.e., parasitic inductance) as well as the equivalent series inductance (ESL) of the decoupling capacitor. Parasitic inductance in traditional IC arrangements can be upwards of 100 pH or more while the equivalent series inductance of the decoupling capacitor can be upwards of 400 pH or more.
One problem with the conventional arrangement of a semiconductor package is that the impedance sensitivity of the power delivery network is affected by the total inductance experienced. For example, there is a strong resonance peak at around a frequency of one hundred megahertz caused by the total inductance experienced by the power delivery network. As the transistors in the IC begin switching, current needs to be supplied by the power delivery network. Because of the current flow through the impedance of the power delivery network, the power supply voltage can fluctuate, compromising stability of the power supplied to the ICs. Compromised stability can result in degraded operation of the IC.
Thus, there is a need for improved apparatuses and methods for reducing the total characteristic impedance in semiconductor packages.